An organic light emission phenomenon is an example of converting current into visible rays through an internal process of a specific organic molecule. The principle of the organic light emission phenomenon is based on the following mechanism. When an organic material layer is disposed between an anode and a cathode, if voltage is applied between the two electrodes, electrons and holes are injected from the cathode and the anode, respectively, into the organic material layer. The electrons and the holes which are injected into the organic material layer are recombined to form an exciton, and the exciton is reduced to a bottom state to emit light. An organic light emitting diode using this principle may typically comprise a cathode, an anode, and an organic material layer, for example, an organic material layer comprising a hole injection layer, a hole transporting layer, a light emitting layer, and an electron transporting layer, disposed therebetween.
The materials used in the organic light emitting diode are mostly pure organic materials or complexes of organic materials with metals, and may be classified as a hole injection material, a hole transporting material, a light emitting material, an electron transporting material, or an electron injection material, according to their use. In connection with this, an organic material having a p-type property, which is easily oxidized and is electrochemically stable when it is oxidized, is usually used as the hole injection material or the hole transporting material. Meanwhile, an organic material having an n-type property, which is easily reduced and is electrochemically stable when it is reduced, is usually used as the electron injection material or the electron transporting material. As the light emitting layer material, an organic material having both p-type and n-type properties is preferable, which is stable when it is oxidized and reduced. When an exciton is formed, a material having high light emitting efficiency for converting the exciton into light is preferable.
In addition, it is preferred that the material used in the organic light emitting diode further has the following properties.
First, it is preferred that the material used in the organic light emitting diode has excellent thermal stability. This is due to joule heat generated by movement of electric charges in the organic light emitting diode. NPB, which has currently been used as the hole transporting layer material, has a glass transition temperature of 100° C. or less, and thus it is difficult to apply NPB to an organic light emitting diode requiring a high current. Also, in order to improve the service life of the organic light emitting diode, the stability of the material itself is important, and since the OLED diode is a diode which provides electricity to generate light, the stability for electric charges is important. This means that when a phenomenon in which electrons are introduced into or emitted from a material is repeated, the material itself is not modified or broken.
Second, in order to obtain an organic light emitting diode that is capable of being driven at low voltage and has high efficiency, holes or electrons which are injected into the organic light emitting diode need to be smoothly transported to a light emitting layer, and simultaneously the injected holes and electrons need to be prevented from being released out of the light emitting layer. To achieve this, a material used in the organic light emitting diode needs to have a proper band gap and proper HOMO and LUMO energy levels. A LUMO energy level of PEDOT:PSS, which is currently used as a hole transporting material of an organic light emitting diode manufactured by using a solution coating method, is lower than that of an organic material used as a light emitting layer material, and thus it is difficult to manufacture an organic light emitting diode having high efficiency and a long service life.
Moreover, the material used in the organic light emitting diode needs to have excellent chemical stability, electric charge mobility, and interfacial characteristic with an electrode or an adjacent layer. That is, the material used in the organic light emitting diode needs to be minimally deformed by moisture or oxygen. Furthermore, a proper hole or electron mobility needs to be assured so as to balance densities of the holes and of the electrons in the light emitting layer of the organic light emitting diode to maximize the formation of excitons. Additionally, it needs to be able to have a good interface with an electrode comprising metal or metal oxides so as to assure stability of the diode.